On 12/13/2012 05:42 PM, Andy Lutomirski wrote:
>
> The 64-bit/x32 case is currently very simple and fast because it uses
> absolute addressing. Admittedly, pcrel references are free, so
> changing this wouldn't cost much. For native, it'll be slower, but
> maybe no one cares. I seem to care about this more than anyone else,
> and I don't use 32 bit code. :)
>
pcrel is actually cheaper than absolute addressing in 64-bit mode.
> The benefit of switching is that the vdso code could be the same in
> all three cases. (Actually, it's even better than that. All of the
> VVAR magic could be the same in the vdso and the kernel -- the kernel
> linker script would just have to have an appropriate symbol to see the
> appropriate mapping.)
>
>
> This:
>
> __attribute__((visibility("hidden"))) int foo;
>
> int get_foo(void)
> {
> return foo;
> }
>
> generates a rip-relative access on 64 bits and GOTOFF on 32 bits.
>
> The only reason I didn't use a real symbol in the first place is
> because I couldn't figure out how to get gcc to emit an absolute
> relocation in pic code.
Well, then, we wouldn't need to do that... this is starting to sound
like a significant win.
-hpa
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